FIG. 1 shows an optical lithographic fabrication system 100 for delineating features in a workpiece 120, in accordance with prior art. Typically the workpiece 120 comprises a semiconductor wafer (substrate), together with one or more layers or substances (not shown) located on a top major surface of the wafer.
More specifically, typically substantially monochromatic optical radiation of wavelength .lambda. emitted by an optical source 106, such as a mercury lamp, propagates successively through a pinhole aperture in an opaque screen 105, an optical collimating lens or lens system 104, a patterned lithographic mask or reticle 103 having a pattern of features in the form of apertures (bright regions) in an opaque material, and an optical focusing lens or lens system 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the workpiece 120. Thus the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a development process, typically a wet developer, the material of the photoresist is removed or remains intact at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101.
Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the workpiece 120 typically comprising a semiconductor wafer. Thus, portions of the workpiece 120 are removed from the top surface of the workpiece 120 at areas underlying those where the photoresist layer 101 was removed by the development process but not at areas underlying those regions where the photoresist remains intact. Alternatively, instead of etching the workpiece, impurity ions can be implanted into the workpiece 120 at areas underlying those where the photoresist layer was removed by the development process but not at areas underlying where the photoresist remains. Thus, in any event, the pattern of the mask 103--i.e., each feature of the mask--is transferred to the workpiece 120 as is desired, for example, in the art of semiconductor integrated circuit fabrication.
As known in the art, the aperture 15 is located on the focal plane of the collimating lens 104, and the indicated distances L1 and L2 satisfy in cases of a simple lens 102: 1/L1+1/L2=1/F, where F is the focal length of the lens 102.
In fabricating integrated circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to make transistor sizes as small as possible. Similarly it is desirable to make as small as possible any other feature size, such as the feature size of a metallization stripe--i.e., its width--or of an aperture in an insulating layer that is to be filled with metal, for example, in order to form electrical connections between one level of metallization and another.
According to geometric optics if it is desired to print on the photoresist layer 101 the corresponding feature having a width equal to W, a feature having a width equal to C must be located on the mask (reticle) 103. Further, according to geometric optics if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, and where m is known as the "lateral magnification". When diffraction effects become important, however, instead of a sharp black-white image a diffraction pattern of the object feature C is formed on the photoresist layer 101, whereby the edges of the image become indistinct. Consequently, the resolution of the features of the reticle 103, as focused on the photoresist layer and transferred to the workpiece, deteriorates.
In prior art this diffraction problem has been alleviated by such techniques as using phase-shifting portions ("phase-shifting features") on the mask. Therefore the mask is then known as a "phase-shifting mask"--and hereinafter the mask 103 will therefore likewise be called "the phase-shifting mask 103". These phase-shifting features impart a phase shift .phi. to the optical beam emanating from the optical source 106, with .phi. typically equal to approximately .pi.(=180.degree.).
Phase-shifting features can be either opaque (i.e., with an optical intensity transmission coefficient, T, approximately equal to 0) or partially transparent (i.e., with the value of T advantageously being in the approximate range of 0.05 to 0.15, typically T being approximately equal to 0.10)--all as measured with respect to the optical radiation of wavelength .lambda.. Thus, in a phase-shifting mask, the features such as that shown in the form of a simple aperture C in the mask 103 will become more complicated than just simple apertures and will include the above-mentioned phase-shifting features (not shown).
Typically, for use in the system 100, the phase-shifting mask 103 has two basic areas: (a) feature areas, and (b) alignment areas. That is to say, the phase-shifting mask 103 has the following basic areas: (a) feature areas whereby the images formed by them on the photoresist layer 101 correspond to device features of the workpiece 120 (such as selected areas of the workpiece 120 where impurity ions are implanted or selected areas of the workpiece 120 where portions of the workpiece 120 are removed), and (b) workpiece fabrication alignment areas thereof whereby the images formed on the photoresist layer 101 correspond to workpiece fabrication alignment marks.
As known in the art, in a step-and-repeat movement procedure for forming images on the photoresist layer 101, the workpiece 120 is subdivided into chip ("die") subregions: each subregion typically is defined and encompassed by one resulting step-and-repeat position of the workpiece 120 (overlain by the photoresist layer 101). Each corresponding subregion of the photoresist layer 101 is successively exposed to the optical beam in the system 100. In order to align the system 100 for the step-and-repeat process, step-and-repeat alignment marks are required on the mask 103 in addition to the workpiece alignment fabrication marks.
As further known in the art, in the system 100 an unwanted diffraction of optical radiation causes an undesirable leakage of optical radiation from one area on the photoresist layer 101 to another. During a step-and-repeat procedure, this leakage will expose areas of the photoresist layer 101 overlying adjacent chip subregions of the workpiece 120 to an undesirable optical radiation background if the adjacent chip subregions are located in close proximity. Therefore this leakage causes undesirable deterioration of the sharpness of definition of features of adjacent chip subregions. In addition to this (step-and-repeat) leakage, an unwanted optical radiation leakage from the feature areas to the alignment areas on the photoresist layer 101 undesirably can reduce the sharpness of definition (contrast ratio) of the images of the alignment marks on the photoresist layer 101; hence, this leakage can cause a deterioration of the sharpness of definition of the positions of the features ultimately formed in the workpiece 120.
To minimize these optical radiation backgrounds--instead of increasing the distance between chips (as defined by the step-and-repeat procedure) or the distance between feature areas and alignment areas, whereby precious feature area on the workpiece 120 would be sacrificed--in prior art an opaque shutter layer is introduced on the mask 103 between the alignment areas and the boundaries of the chip subregions, as well as between the feature areas and the alignment areas. In the interest of economy of processing, workers in prior art formed these shutter layers simultaneously with forming the opaque layers (T=0) of the phase-shifting areas. However, in cases where the phase-shifting features are partially transparent (i.e., in cases where T.noteq.0), such a method would produce alignment marks and shutter layers that would likewise be partially transparent, whereby the alignment of the photoresist layer 101 during the step-and-repeat procedure as well as during exposure to the optical beam in the system 100 would be compromised, and an undesirable amount of leakage would persist.